This invention concerns a liquid treatment apparatus in which a plurality of treatment beds are housed in a vessel through which a liquid to be treated is passed. More specifically, it is concerned with a tank structure for use with volume variable treatment beds.
A major use of this type apparatus is in the demineralization of water using beds of ion exchange resins and this description will largely deal with the use of the invention as a demineralizer. However, the invention may be used in other types of liquid treatments with other kinds of volume variable liquid treatment beds. These will be discussed later in the application.
There are three common systems presently used for demineralization of water (excluding distillation): the separate tank system, the mixed bed system and the multiple bed system.
In the separate tank system, cation exchange resin is contained in one vessel and anion exchange resin in another separate vessel. Each vessel has a top collector/distributor and a bottom collector/distributor and usually another collector/distributor in the top layer of resin, or just above it for the addition and removal of water, and of backwash water, chemical solutions, and resins. Each vessel has its own associated piping valves, controls and necessary regeneration equipment. The vessels are connected in series. Water flows through the cation exchange vessel, then through the anion exchange vessel, and when the ion exchange resins are exhausted, the vessels are taken out of service, the resins regenerated and then returned to service. The ion exchange resins can be regenerated with a fluid flow in the same direction as the normal water flow, i.e. concurrent regeneration, or the regeneration may be performed in a direction opposite to the normal water flow, counter-curent regeneration. The best quality of effluent demineralized water is achieved when counter-current regeneration is performed. A major disadvantage with this type of a system is the cost involved in providing each bed with its own tank each having its own pipes, valves and controls.
In the mixed bed system, the cation and anion exchange resins are thoroughly mixed together, usually by air agitation, and contained in one vessel. The water passes through the mixed resins, and usually excellent quality demineralized water is produced. The vessel contains top and bottom collector/distributors and a structurally very strong and expensive conduit system for the collection or distribution of fluids fixed in place at the position where the cation and anion resins are supposed to separate (by backwashing) prior to regeneration, and often another collector/distributor just above the top of the ion exchange bed. There are external pipes, valves, controls, and regeneration equipment. Water flows down through the mixed bed resins until the resins are exhausted and then the vessel is taken off line for regeneration. Water is introduced into the bottom of the vessel and flows upwards through the vessel and this carries the lighter resin to the top of the bed thus separating the resins. The separation is supposed to occur where the central conduit system is located. Separate regeneration of the cation and anion resins is then conducted using the central conduit system in treating individually first the cation resin above and then the anion resin below it. After this the resins are thoroughly mixed and the vessel is returned to service. A problem arises, however, if the resin separation does not occur at the level of the central conduit system.
To avoid the disadvantages of these two systems a third system has been used: a multiple bed apparatus in which the cation and anion beds are situated in one tank separated by a fluid permeable partition to prevent their intermixing. Usually this partition is mounted in a fixed position on the tank walls, a disadvantage in a tank containing volume variable beds of ion exchange resins which expand and contract as they are exhausted and then regenerated. An expanding bed would put pressure on the immovable bed partition which could do structural damage to the apparatus. The U.S. Pat. Nos. to Miller, 3,554,377 and Lindenthal, 3,497,069, disclose two approaches to this problem. In Miller, the various beds are separated by longitudinally movable filters which prevent intermixing of the beds while being free to move longitudinally in response to bed compaction caused by the pressure of the liquid flow. In Lindenthal, the respective beds are separated by resilient foam partitions which expand and contract to compensate for changes in volume of the beds.
Multiple bed treatment devices present an additional problem encountered during regeneration of the beds analgous to the fixed conduit system problem of the mixed bed system. Since each bed has a different chemical makeup, each requires a different chemical treatment to be regenerated. Prior art devices have approached this problem by positioning fluid conduit distribution and collection systems between the beds so that the beds can be treated individually. These conduit systems are, however, as is so often the case with bed partitions, mounted in fixed positions relative to the tank. Since they cannot move with the volume changes of the respective beds, they must necessarily be of extra strong construction to resist the volume changes. Also, a volume change in the beds may cause a shift of material moving the interface of a pair of beds away from the fluid conduit system. No provision has been made for devices having one or more volume variable beds each requiring individual treatment combining the advantage of a moving bed separator which adjusts to volume changes with a between-bed fluid conduit system for the individual treatment of beds.